In automatic shifting power transmissions it is common practice to install a hydrodynamic fluid drive, such as a torque converter, between the power source (engine) and a multi-speed gear configuration, such as a planetary gear arrangement. The torque converter provides a torque multiplier between the engine and the gearing to improve the vehicle launch performance. The torque ratio of the torque converter is generally in the range of 1.60 to 3.3 depending on the particular application. As is well-known, the torque converter is a slipping device that has a high efficiency loss at vehicle launch. This loss decreases, but continues, as the torque converter approaches a 1.0 to 1.0 speed ratio at high speed and low torque. In recent times, a torque converter lock-up has been added to most transmissions to effectively remove the torque converter from the power path and thereby improve the overall efficiency of the transmission.
Other considerations have been given to improving the overall efficiency of the transmission. Specifically, newer automatic transmissions, with their higher number of gear ratios and higher overall ratio, have less need for torque converter multiplication. This reduced need for torque multiplication has lead to the use of a “friction launch” device in place of a torque converter within stepped gear automatic transmissions. For example, the use of a starting clutch in lieu of the torque converter has been proposed. The advent of electronic controls improves the operation of a starting clutch as a vehicle launch device. The starting clutch is, however, a rotating torque transmitting device which must still deal with all of the complexities associated with such a vehicle launch device. Thus, the control needs considerable accuracy to insure consistent fill times, and to compensate for variable fluid leaks at the rotating shaft seals. This requires accurate hydraulic flow volumes and pressure control over a wide range of operating requirements. Also the use of a starting clutch merely replaces one rotating mechanism with another, albeit a more efficient mechanism. There is only slight axial space saving and perhaps more complex control features.
As noted, if friction launch is applied to a conventional automatic transmission, the starting clutch must fulfill the same requirements as the torque converter. These requirements include, for example, shift and launch quality, NVH and driveline isolation, mass, peak acceleration, and durability. Many of these and other requirements apply to both forward and reverse launches.
The most common implementation of friction launch is simply to locate the starting clutch between the engine and the input to the transmission. This implementation has been used in production with manual transmissions, automated manual transmissions, and continuously variable transmissions. One advantage of using a stand-alone starting clutch assembly is that it can be designed as a drop-in replacement for the torque converter of an existing automatic transmission. Stand-alone starting clutches also have the ability to isolate the transmission from torsional engine inputs, which is an advantage compared to other embodiments which locate the starting clutch further downstream in the powerflow. Another advantage is that the starting clutch can be designed for a lower torque capacity than some of the other implementations. Another variation of a dedicated launch is to locate the launch clutch on the output of the transmission. One of the benefits of an output clutch is the ability to use the clutch as a “fuse” between the transmission and road inputs, thus allowing the clutch to slip at certain load levels and protect the remainder of the transmission.
When applying friction launch to a stepped gear automatic transmission, greater benefits in cost, mass, and packaging can be achieved if an existing clutch can be re-used as the launch clutch. Preferably, the powerflow will allow the same input clutch to be used for both forward and reverse launches. Greater benefits in cost, mass, and packaging can be achieved if the same clutch can be used for both forward and reverse launches.
Another way to apply friction launch to a stepped gear automatic transmission would be to re-use one or more existing reaction clutches for launch. The use of a reaction clutch offers possible cooling advantages and eliminates the need for centrifugal compensation compared to a rotating clutch. Again, the greatest benefits can be achieved if the same reaction clutch can be used for both forward and reverse launch. It is also possible to re-use a combination of an existing input clutch and an existing reaction clutch for forward and reverse launch. One example of such an automatic transmission is disclosed in commonly-owned U.S. Pat. No. 6,471,616.